Main candidates for the precursors of the proto-Greeks in the ancient DNA record to date

Thanks to the recent release of the Mathieson et al. 2018 dataset (see here), I've been able to spot a very interesting northwest to southeast genetic cline running from the oldest Peloponnese Neolithic (Peloponnese_N) individuals to the Bronze Age Anatolians (Anatolia_BA). Here it is, highlighted in my Principal Component Analysis (PCA) of ancient West Eurasian variation. The relevant datasheet is available here.

I don't think it's a stretch to assume that this cline represents, more or less, the genetic diversity that existed in the Aegean region during the early Helladic period, just prior to the incursions of Bronze Age steppe or steppe-derived peoples who, according to the current academic consensus, probably gave rise to the proto-Greeks and Mycenaeans (see here).

There are three main reasons for this: 1) the Peloponnese_N samples show a very deliberate "pull" towards Anatolia_BA, suggesting that the Peloponnese population experienced admixture from a source similar to Anatolia_BA prior to the Bronze Age, 2) the cline cuts right through the middle of an "Old European" cluster made up of Minoans, who lived on Crete and other Aegean islands on the eve of the aforementioned steppe-derived incursions, and 3) both the Mycenaeans and Minoans can be modeled in large part as Anatolia_BA and Peloponnese_N.

The identification of this genetic cline, and what it likely stands for, is important, because it should allow us to plausibly point to the source of foreign input that created the Mycenaeans, and thus the Proto-Greeks. And clearly, the trajectory of the Mycenaean "pull" away from this cline is towards most of the samples marked as "Eneolithic and Bronze Age steppe".

However, this doesn't mean that it's necessary, or even sensible, to look for the precursors of the Proto-Greeks amongst these samples. That's because there might be much more proximate options based on, say, geography, archeology, chronology and mixture modeling. Indeed, using various criteria, I've chosen three individuals who sit along the Mycenaean to Eneolithic/Bronze Age steppe cline in the above PCA and might plausibly represent the precursors of the Proto-Greeks, or close relatives thereof. The first two are from Mathieson et al. 2018 and the third from Olalde et al. 2018.

- if, as most academics posit, the people who were to become the Proto-Greeks came from the Early Bronze Age (EBA) Yamnaya horizon on the Pontic-Caspian steppe, then it's possible that they were similar in terms of genome-wide genetic structure to the only Bulgarian Yamnaya sampled to date: Yamnaya_Bulgaria Bul4

- on the other hand, if, as has also been postulated in academic literature, they derived from the Middle Bronze Age (MBE) chariot warrior groups of the post-Yamnaya Pontic-Caspian steppe, then they may have been similar to Balkans_BA I2163, who is also from Bulgaria, but dated to more than a thousand years later than Bul4, and clusters strongly with the said chariot warriors, such as the Sintashta people, and even belongs to the same Y-haplogroup: R1a-Z93

- but if they came from the Yamnaya horizon via the Carpathian Basin, which, I'm told in the comments here, is also a serious option, although admittedly I've missed it in my reading, then they may have been similar to Proto-Nagyrév individual Hungary_BA I7043, who belongs to Western European-specific Y-haplogroup R1b-L51, a marker fairly common amongst modern-day Greeks.

And here's a mixture model for the Mycenaeans, using the Global25/nMonte method (see here and here), and the above trio as potential reference samples, alongside Anatolia_BA and Peloponnese_N.

[1] distance%=1.9802

Mycenaean

Peloponnese_N,45.2
Anatolia_BA,35.8
Yamnaya_Bulgaria,16.4
Balkans_BA,2.6
Hungary_BA,0

Thus, it seems that the precursors of the Proto-Greeks came from Bulgarian Yamnaya. However, they, or the Mycenaeans, may also have had minor ancestry from the chariot warriors of the MBA Pontic-Caspian steppe. Yes, I'm probably reading far too much into these results, but I can't help it, because they appear so logical. Indeed, check this out:

[1] distance%=4.209

Mycenaean:I9033 (elite burial)

Peloponnese_N,38.8
Yamnaya_Bulgaria,33.4
Anatolia_BA,27.8
Balkans_BA,0
Hungary_BA,0

If this is just an artifact of the method, then it's a really nice one. But who are your main candidates for the precursors of the Proto-Greeks in the ancient DNA record to date? Feel free to let me know in the comments.

See also...

Late PIE ground zero now obvious; location of PIE homeland still uncertain, but...